What is the California Bearing Ratio (CBR)?
The California Bearing Ratio (CBR), as defined in BS 1377-9, is the standard metric used to assess the load-bearing capacity and thickness requirements for roadways, pavements, car parks, runways, and foundations. This essential test evaluates soil strength and ability to support structural loads, making it a cornerstone of road and infrastructure design.
The CBR test involves a standardised penetration method initially developed in 1930 by the California State Highway Department. Since then, it has become a globally recognised technique for assessing subgrade and subbase performance under traffic and other applied loads.
Reliable and Efficient California Bearing Ration (CBR) Testing from GSTL
At GSTL, we offer industry-leading expertise in conducting California Bearing Ratio tests, utilising two specialised types of test frames to suit various project needs.
- Vehicle-Mounted Frames
- JCB- Attachment Frames
Our tests are conducted by skilled technicians equipped with 4-wheel drives or JCBs to transport and operate the required equipment seamlessly, ensuring efficient service delivery to your site.
How Does a CBR Test Work?
The CBR test procedure begins with preparing the test site. Our technicians deploy a cylindrical plunger into the soil at consistent intervals. The plunger measures the pressure required to penetrate a standard soil area, providing critical data about the soil's strength.
Typically, the test is repeated up to 10 times and completed within a day, allowing for rapid reporting of results and helping you meet project deadlines. The data collected enables precise calculations of load capacity and aids in determining optimal pavement thickness and subgrade stability.
What projects require a California Bearing Ratio (CBR) Test?
Roads and Pavements - The CBR test is indispensable for projects involving roads, pavements, or highways. It measures the strength and load-bearing capacity of subbase and subgrade soils, replicating the stresses caused by vehicular traffic. With CBR results, engineers can:
- Assess soil suitability for construction.
- Determine the necessary pavement thickness to ensure durability and performance under traffic loads.
- Identify areas where soil improvement is required.
Foundations - For civil engineering projects, ensuring that the ground's strength, stability, and composition is critical for any foundation. The California Bearing Ratio test provides an accurate evaluation of soil conditions, helping to identify:
- Whether the existing soil can support the structure.
- If stabilisation techniques, such as reinforcement or additional materials, are required.
Why Choose GSTL for CBR Testing?
- Expert Team - Our trained technicians ensure accurate test execution and reliable results.
- Modern Equipment - Advanced vehicle-mounted and JCB-attached frames provide flexibility and efficiency.
- Quick Turnaround - Most tests are completed and reported within a day, minimising delays in your project timeline.
For additional testing requirements, such as working platform evaluations, visit our Plate Bearing Test section to learn more.
Frequently Asked Questions
How do you calculate the CBR ratio?
The CBR value is calculated by comparing the force required to penetrate a soil sample with a standard piston, to the force required to penetrate a similar piston into a standard crushed stone material.
The formula is:
CBR = ( Load required to penetrate soil / Load required to penetrate standard material ) ×100
What is a good CBR value?
A good CBR (California Bearing Ratio) value depends on the intended use of the soil and the requirements of the construction project, such as road design or pavement thickness. Generally, a higher CBR indicates better soil strength and load-bearing capacity, while a lower CBR indicate a weaker soil that may require additional design considerations.
To summarise
- Good CBR values for most construction projects (especially for road construction) typically start around 10% and above.
- For highways or heavy-load pavements, CBR values of 20% or higher are often considered ideal.
- Lower values (below 5%) usually require soil improvement methods like stabilization, grading, or adding higher-quality materials.
Thus, a good CBR value is one that meets the specific requirements of your project, but generally, a CBR value of 10% or higher is often considered acceptable for many types of construction, with values above 20% being considered strong and ideal for road foundations.
How do you test for California Bearing Ratio?
· Set up the CBR testing machine with the penetration piston and dial gauge.
· Apply load gradually and measure the load required to achieve specific penetration depths (2.5 mm and 5.0 mm).
· Calculate the CBR using the load data, comparing it to standard crushed stone values.
· Interpret the CBR values to determine the suitability of the soil for construction.
Is a CBR test the same as a plate bearing test?
No, a CBR (California Bearing Ratio) test and a Plate Bearing test are not the same, though both are used to assess the bearing capacity of soil for construction purposes. These two tests measure different properties and involve different methodologies. Here's a breakdown of the differences between the two:
· The CBR test is more focused on evaluating the load-bearing capacity of soil under penetration conditions, typically in a laboratory setting. It is primarily used in pavement design and road construction, particularly for estimating the strength of soil at various depths.
· The Plate Bearing test, on the other hand, provides a direct measure of the bearing capacity of the soil in the field. It involves applying a load to a steel plate and measuring the settlement under load to determine how the soil will behave under real-world conditions, which is useful for designing foundations and structures.
